Detailed analysis by in vivo and in vitro methods of antitumor drug effects on hematopoietic precursor cells will be continued with emphasis on 2 problems: 1) A re-assessment of the possible specific cytotoxic effects of Myleran on murine pluripotential stem cells and of Actinomycin D on erythropoietin-responsive cells. 2) Potentiation of cytotoxicity of drugs through cell-cycle manipulation by bacterial endotoxin or other agents.